Composite membranes capable of selectively permeating one component of a fluid mixture over the remaining components in the mixture generally include a thin layer or coating of a suitable semipermeable membrane material superimposed over a porous substrate. Generally, while the coating affects the separation characteristics of the composite membrane, the primary function of the substrate is to provide support for the coating positioned thereon. Common porous substrates are configured as flat-sheet membranes or as hollow fibers.
In commercial or industrial applications, composite membranes need to operate for extended periods with low incidence of failure. Furthermore, the membranes often must withstand foul or corrosive environments.
One method of preparing composite membranes is to coat a preformed porous substrate, such as a porous hollow fiber, with a dilute solution of a polymer in a solvent, followed by the removal of the solvent by drying. However, it is generally very difficult to produce defect-free thin-film composites with thicknesses of less than 1 micro meter (1 μm) by the solution coating process. Furthermore, it is generally recognized in the art that it is difficult to produce viable (substantially defect-free) high productivity composite membranes by solution coating of substrates having high surface porosity. In particular, in order to produce complete surface coverage, the coating solution must fully wet the substrate. Capillary forces in a fully wetted porous substrate, however, tend to draw the coating solution into the surface pores leading to an effective increase in separation layer thickness and to a decrease in membrane productivity. This is often referred to as occlusion of the pores by the coating solution.
Several processes for making composite membranes are known in the art.
U.S. Pat. No. 4,840,819, to Williams et al., discloses a process in which a dilute solution of permeable polymer is applied to a porous substrate having a controlled amount of liquid incorporated therein.
U.S. Pat. No. 4,806,189, to Kraus et al., discloses a process for producing a composite fluid separation membrane by in situ formation of a separation layer on a porous support wherein the pores of the support are pre-impregnated with a solvent.
U.S. Pat. No. 5,320,754, to Kohn et al., discloses preparation of composite membranes by applying perfluoroethers to the surface of a porous substrate prior to coating with a selective polymeric material.
U.S. Pat. No. 5,213,689, to Kafchinski et al., discloses a method of coating microporous polyolefin hollow fibers by wet spinning or by dry-wet spinning. Polyolefin hollow fibers are coated with SIXEF™-Durene polyimide containing perfluoro groups from the solvent NMP. The polyolefin hollow fiber is optionally pre-wetted with glycerine prior to coating.
M. Rezac et al. in the Journal of Applied Polymer Science, V46, p. 1927(1992), teach preparation of composite membranes from solutions of ultra-high molecular weight polymers. The authors suggest that improved membranes are formed when polymer chain dimensions of the coating material are larger than the surface pores of the porous support.
Several amorphous perfluoropolymers have been used as coating or membrane materials, including perfluoropolymers with high gas permeation characteristics.
U.S. Pat. No. 5,051,114, to Nemser et al., discloses amorphous perfluoro-2,2-dimethyl-1,3-dioxole based polymers that can be used for several separation and gas enrichment applications, including oxygen enrichment of air.
U.S. Pat. No. 4,754,009, to Squire, discloses a gas permeable material that contains passageways wherein the interior of the passageways is formed by solution coating of perfluoro-2,2-dimethyl-1,3-dioxole.
U.S. Pat. No. 5,876,604, to Nemser et al., discloses the preparation of composite perfluoro-2,2-dimethyl-1,3-dioxole membranes that can be used to add a gas to a liquid or to remove a gas from a liquid. The membranes exhibit resistance to fouling by liquids, and can be utilized for ozonolysis or oxygenation.
U.S. Pat. No. 5,914,154, to Nemser, discloses preparation of non-porous gas permeable membranes by flowing a dilute coating solution of perfluoropolymer through one side of a microporous substrate, until the desired thickness of coating polymer is built up; the solution is then removed and residual solvent is evaporated.
Existing processes for producing composite perfluoropolymer membranes result in relatively thick coating layers, believed to be due, at least in part, to intrusion of the coating layer into the porous support. The resulting composite gas separation perfluoropolymer membranes exhibit relatively low gas permeance.
Therefore, a need exists for high productivity composite membranes and processes for making them in which these problems are eliminated or reduced.